The disclosure relates generally to turbomachines such as steam and gas turbines, and more particularly, to an apparatus and method for obtaining discrete axial clearance data using radial clearance sensors.
Turbomachines, such as gas and steam turbines, typically include a centrally-disposed rotor that rotates within a stator. A working fluid flows through one or more rows of circumferentially arranged rotating blades that extend radially outward from the rotor shaft. The fluid imparts energy to the shaft, which is used to drive a load such as an electric generator or compressor.
Clearance between radially outer tips of the rotating blades and stationary shrouds on an interior of the stator strongly impacts efficiency of a turbomachine. The smaller the clearance between the rotor blades and the inner surface of the stator, the lower the likelihood of fluid leakage across blade tips. Fluid leakage across blade tips causes fluid to bypass a row of blades, reducing efficiency. However, insufficient clearance is also problematic. Operating conditions may cause blades and other components to experience thermal expansion at different rates, which may result in variations in blade tip clearance. The specific effects of various operating conditions on blade clearance may vary depending on the type and design of a particular turbomachine. For example, tip clearances in gas turbine compressors may reach their nadir values when the turbine is shut down and cooled, whereas tip clearances in low pressure steam turbines may reach their nadir values during steady state full load operation. If insufficient tip clearance is provided when the turbomachine is assembled or re-assembled after inspection/repair, the rotating blades may hit the surrounding shroud, causing damage to the shroud on the stator interior, the blades, or both when operating under certain conditions.
Radial clearance in a turbomachine, e.g., a steam turbine, can be measured with hermetically sealed clearance sensors positioned on a stationary component. For example, the sensors can measure the gap between a tip of the sensor and a point on the rotating component by using sensors that have a voltage output that is indicative of the gap. The clearance can be determined by correlating voltage data, steam capacitance, and gap distance. During turbine operation, the rotating component and the stationary components will likely heat and cool at different rates, and therefore, the axial position of the clearance sensor, located on the stationary component, will move relative to the rotating component. Because of this relative movement, relatively large sensors with wide sensor tips are desired, to accommodate the axial movement. However, large sensors are often difficult to include in the relatively small areas available on the stationary component. Therefore, in practice, sensors are usually smaller than desired, and the entire range of differential axial motion is not covered. This results in a loss of signal, indicating that the differential rotor-stator axial movement has exceeded the axial range of the clearance sensor operation.